1. Field of the Invention
This invention relates to an electromotive adjustable resistor, and particularly to an adjustable resistor which is used primarily in a fixed position on the circuit board of an electronic apparatus. Resistance of the adjustable resistor is changed by rotating its resistance adjusting shaft by electromotive force or manually.
2. Discussion of Related Art
A conventional electromotive adjustable resistor 20 is illustrated in FIGS. 4, 5 and 6 which will be described herein.
As shown in FIG. 5, a gear case 2 is covered by a steel plate 1. An adjustable resistor 3, having a resistance adjusting shaft 4 extending to both sides of the adjustable resistor 3, is fixed on the steel plate 1. A driving component 6 having a circular portion 10 and a pillar portion 7 is fixed by a speed nut 5 on the resistance adjusting shaft 4 which is extending into the gear case 2. A second worm wheel 9 is assembled concentrically around the resistance adjusting shaft 4 and biased to the driving component 6 by a spring 8. The second worm wheel 9 and the driving component 6 are coupled fictionally to permit the transmission of the rotating torque between them. A pad 11 is inserted between the second worm wheel 9 and the driving component to stabilize and to reinforce the frictional coupling between them. A first worm gear 13 is placed on a motor shaft 12A of an electric motor 12.
As shown in FIG. 6, which is a sectional view taken along line A--A of FIG. 5, a gear shaft 19 is assembled rotatively around an axis C--C, having a first worm wheel 14 and a second worm gear 15 fixed on the axis C--C. The rotation of the motor shaft 12A is transmitted to the second worm wheel 9 via the first worm gear 13, the first worm wheel 14, and the second worm gear 15.
As is further shown in FIG. 5, the rotating torque of the second worm wheel 9 is transmitted to the driving component 6 through the friction coupling between them. This causes the rotation of the resistance adjusting shaft 4, so that the resistance of the adjustable resistor 3 is changed.
When supply current of the electric motor is cut off, the resistance of the adjustable resistor 3 can be manually changed by rotating the resistance adjusting shaft 4, which is extending to the counter side of the gear case 2. The driving component 6 rotates with the resistance adjusting shaft 4, but the second worm wheel 9 does not rotate with the driving component 6, since the second worm wheel 9 is prevented from rotation in both directions by the engaged second worm gear 15. This is accomplished by slipping the driving component 6 against the second worm wheel 9 through the pad 11 inserted between them.
The second worm wheel 9 and the first worm gear 13 are both arranged in the same plane in order to make the gear case 2 smaller, so that the distance between the centers of the second worm wheel 9 and the first worm gear 13 is larger than the sum of the radiuses of these components as shown in FIG. 6. This controls the width W of the conventional electromotive adjustable resistor 20, as shown in FIG. 5, making it of a comparatively large size, and prevents one from making the width narrower.
Accordingly, the conventional electromotive adjustable resistor 20 occupies a relatively large space when it is positioned on the circuit board 16. The electromotive adjustable resistor 20 is connected to the circuit board 16 by connecting an electric connecting terminal 17 of the adjustable resistor 3 and an electric connecting terminal 18 of the electric motor 12 to the circuit board 16, as shown in FIG. 4.
Thus, the electromotive adjustable resistor described above does not provide the important advantages of having an electromotive adjustable resistor constructed to be more resistant to mechanical vibration and occupying smaller space on the circuit board due to its improved construction of the gear case and the internal gear train.